How low should I bend my torso for a bent over bb row? 45 degrees or 90 degrees? I've been doing 45 degrees for some time but recently read that you should do it with your torso at 90 degrees or close to it. Any ideas?

There's a thread just below this one on rows where VeganEssentials wrote:

"Also, with barbel rows, you'll find less strain if you do them with the torso at about 45 degrees vs. parallel to the floor and by keeping the bar as close to the body as possible. It will activate the upper back muscles slightly differently than a wider grip pulling to the middle chest (more latissimus activation vs. other middle back/rear deltoid work), but is still a useful way to row and tends to be less stressful on the lower back."

So it depends on which muscle group you are more interested in working, and whether you feel any lower back strain standing with your back 90 degrees to the floor (as I was). And your knees should be bent, yes.